Generally on a vehicle, a battery is mounted as a power source of a load. Japanese Patent Laying-Open No. 2005-14707 discloses a state monitoring device for a vehicle-mounted battery capable of performing battery state monitoring while minimizing a current consumed during an engine stop period. This state monitoring device stores map information indicating a correlation between a charging rate of the battery and a terminal voltage (pseudo open voltage) when a charging/discharging current value of the vehicle-mounted battery is within a very narrow prescribed range. Furthermore, this state monitoring device stores a first charging rate SOC1 and a first remaining capacity SOH1 of the battery when the engine is stopped. When the charging/discharging current value while the engine is not operated is within the very narrow prescribed range, the state monitoring device periodically detects the terminal voltage (that is, pseudo open voltage) of the battery, and calculates a second charging rate SOC2 in accordance with the detected pseudo open voltage. The state monitoring device calculates a current remaining capacity SOC2 from first charging rate SOC1, first remaining capacity SOH1 and second charging rate SOC2. By setting a time interval between detection of the terminal voltage of the battery to, for example, about several hours, the current consumed by the state monitoring device can be reduced.
Japanese Patent Laying-Open No. 2005-14707 discloses that the state of charge of the battery after startup of the engine is calculated based on the discharging current and the terminal voltage of the battery. In a case where the battery is connected to the load, the terminal voltage of the battery changes from the open voltage and eventually stabilizes at a certain voltage. For example, in a case where the load has a large capacity, however, it takes a certain amount of time to stabilize the terminal voltage of the battery after the battery is connected to the load. Therefore, it is difficult to find accurate SOC before the terminal voltage of the battery is stabilized. Furthermore, in a case where SOC obtained at this time has low accuracy, it is also difficult to increase the accuracy of the result obtained by calculating SOC in accordance with charging/discharging of the battery.